pgsql: Fix combo_decrypt() to throw an error for zero-length input when

Log Message:
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Fix combo_decrypt() to throw an error for zero-length input when using a
padded encryption scheme.  Formerly it would try to access res[(unsigned) -1],
which resulted in core dumps on 64-bit machines, and was certainly trouble
waiting to happen on 32-bit machines (though in at least the known case
it was harmless because that byte would be overwritten after return).
Per report from Ken Colson; fix by Marko Kreen.

Tags:
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REL7_4_STABLE

Modified Files:
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    pgsql/contrib/pgcrypto:
        px.c (r1.7 -> r1.7.6.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/px.c?r1=1.7&r2=1.7.6.1)